1. Field of the Invention
The present invention relates to an optical time domain reflectometer which is used in an optical fiber network to inspect for damage of the optical fibers.
2. Prior Art
The optical time domain reflectometer (OTDR) is an apparatus which measures a damaged point, a transmission loss or a connecting loss of an optical fiber by outputting a light pulse from the OTDR to a measuring optical fiber through an optical waveguide directional coupler, and by detecting a returned light pulse from the measuring optical fiber.
FIG. 4 is a block diagram showing a conventional OTDR. In FIG. 4, a timing pulse generating portion 1 generates an electrical pulse. A driver circuit 2 generates a pulse current according to the electrical pulse, and outputs the pulse current to a light source 3. The light source 3 generates a light pulse (laser) according to the pulse current. The light pulse is inputted to a measuring optical fiber 10 through an optical waveguide directional coupler 4. As a result, light such as back scattering light and reflected light is returned from the optical fiber 10 to the optical waveguide directional coupler 4. The returned light is transmitted from the optical waveguide directional coupler 4 to a light receiver 5. The returned light is then transformed to an electrical signal in a light receiver 5, and the electrical signal is amplified in an amplifier 6. The electrical signal from the amplifier 6 is transformed into a digital signal, and an operation, such as an averaging operation, is applied to decrease a noise in the signal in the digital operating portion 7. Furthermore, the digital signal is transformed on the basis of a logarithm, and is then displayed in the display portion 8.
The back scattering light from the measuring optical fiber 10 is caused by a raleigh scattering light which accrues in the measuring optical fiber 10. The level of the back scattering light becomes approximately 50 dB lower value than the level of the inputted light pulse, in the case wherein the measuring optical fiber 10 is a single mode fiber, and the inputted light pulse has a width of 1.times.10.sup.-6 second.
The velocity of the light pulse, when it is transmitted in the optical fiber, is the value which is used to divide an evacuated light velocity with a refractive index of a material of the optical fiber, and thereby the extent of the light purist in a length direction (along the long axis) of the optical fiber is obtained by producing the velocity and the pulse width of the light pulse. In this embodiment, the extent of the light pulse becomes approximately one hundred meters in the case of the single mode fiber.
In the OTDR, the extent of the light pulse in the length direction (along the long axis) must be a small value in order to improve the resolving power for measurement of the distance to a damaged point. For example, if the inputted light pulse has a width of 1.times.10.sup.-8 second, the extent thereof will be approximately one meter. However, the level of the back scattering light becomes a low value in proportion to the pulse width, that is, the level of the back scattering light becomes approximately 70 dB lower in value than that of the inputted light pulse.
Furthermore, a cross talk light (L1), which occurs in the optical waveguide directional coupler 4, is directly transmitted to the light receiver 5. A reflected light (L2), which occurs at an input connector of the measuring optical fiber 10 via an outputted light from the optical waveguide directional coupler 4 is refracted, and is transmitted to the light receiver. This cross talk light (L1) and reflected light (L2) are supplied to the light receiver 6 with the back scattering light.
The level of the cross talk light (L1) is determined by the efficiency of the optical waveguide directional coupler 4, and, in this case, is approximately 40 dB lower value than that of the inputted light pulse. The level of the reflected light (L2) is determined by the efficiency of the connector, and becomes approximately 40 dB lower in value than that of the inputted light pulse.
The cross talk light (L1) and the reflected light (L2) are supplied to the light receiver 5 at approximately the same time, so that both are added, and thus the level of light inputted to the light receiver 5 is two times the value thereof.
However, in the conventional OTDR, the light receiver 5 must receive only the back scattering light which is approximately 70 dB lower than the inputted light pulse. However, the light receiver 5 receives the sum of the cross talk light (L1) and the reflected light (L2), which is approximately 40 dB lower than the inputted light pulse (i.e. one thousand times the intensity of the back scattering light), with the back scattering light. As a result, the light receiver 5 and the amplifier 6 are saturated. An interval whereat the light receiver 5 can normally receive the back scattering light after the above mentioned effect decreases, is called a dead zone.
The level of the back scattering light, the level of the cross talk light(L1), the level of the reflected light (L2) and the level of the sum of the cross talk light (L1) and the reflected light (L2), in the case wherein the width of the light, pulse is 1.times.10.sup.-8 second, and the dead zone NT are shown as a display example of the display of the OTDR in FIG. 5. As shown in FIG. 5, the conventional OTDR has a relatively long dead zone NT, add as a result, it is not possible to detect damages to the optical fiber quickly.